This post's title was the single line answer given by my young son in a recent science test. He was surprised that it earned him an unsatisfactory mark. After all, his answer was factually correct. His teacher's hint to his failing was that he had not elaborated on the tools he had used to derive his answer. His retort was that he was given no tools to detail.
What is needed, of course, is observation to back up the hypothesis. This is not necessarily that which you see—observe with your eyes—since after all you can't see hardness. You can, however, feel hardness, that is to the extent that it is not soft. But there's not much dynamic range in using your fingers to distinguish between two hard objects such as a rock verses a table. Meanwhile, this has all been predicated on the question of a definition of hardness. When I claimed that it was the lack of softness, this gave us a qualitative scale but not one we could use to distinguish between two different "hard" objects. So how are we to distinguish between two different hard objects using the tools we have at hand in an elementary school classroom? The hint lies in providing a clear statement of the hypothesis by defining "hard" rigorously, and then determining measurements to back up a hypothesis such as the rock is harder than my head.